tools: add a dm-logwrites replay tool

Trying to decipher a dm-logwrites replay failure from generic/482 is
difficult. The test destroys all the dm state and devices when the
test fails, leaving nothing behind for post-mortem analysis. This
script is intended to allow replay of the dm-logwrites device one
FUA at a time similar to generic/482, and is used in conjunction
with a modifed g/482 test that does not tear down the dm volumes
or state when the test fails.

This allows the developer to replay the logwrites up to just prior
to failure to examine just the differences between the last good
state and the first failure state, greatly reducing the complexity
of analysing failures.

Instructions for use are in the tools/dm-logwrite-replay script
itself.

Signed-off-by: Dave Chinner <dchinner@redhat.com>